Servo - January 2008

To make a change to an individual parameter, read both of the values stored at the checksum and the parameter EEPROM locations. Calculate the difference between the new parameter value minus the old parameter value, then subtract this value from the checksum value. Write these two new values to the EEPROM. Remember, if the checksum value becomes negative, it needs to roll over to 255 again.

For example, if I wanted to change the Servo ID from its default value of 0x00 to 0x0A ( 10 decimal), then I can write a program to read the two EEPROM locations as shown here using the 0xE1 command. (Note the values shown are from the HSR-5980SG servo). Then I make the overall checksum change calculation, also shown in Table 3.

These two new values are then programmed back to the same EEPROM addresses using the 0xE2 command as shown. Remember, the checksum value calculated above

is the overall EEPROM checksum value, which is sent as Param2, and is not the same checksum value that is computed for each command packet sent to the servo as described earlier (see Table 4).

In order for these parameter changes to take effect, the servo must be powered down and restarted. If there was an error in calculating the checksum, the servo will still work but the motor will be disabled until the problem is corrected.

If for some reason the servo stops functioning properly due to programming problems, the HMI Servo Programmer software has a servo reset feature that will reset the servo back to the factory defaults. All you need to do is start the program, select the servo type, and select “Servo Reset” under the File menu item. Then press the Yes button, and the servo will be reset back to its original condition.

At this point, you should have all the information needed to control these servos using serial communica-

tions. By monitoring the data being transmitted between the servo and the computer (using a program like the Free-Serial-Port-Monitor), the specific parameters and their functions can be determined. This information can then be modified to change your servo performance characteristics.

At some point, someone will take the time and, using the steps presented here, compile a complete list of all of the parameters and their functions for the different HSR servos and publish them to make our lives easier.

At 19200 baud, it will take about 4 ms of time to send the seven byte data package. This takes about twice as long to command a servo than using the traditional pulse width method. But the command only needs to be sent once, the 20 ms servo update is no longer needed, velocity is now a control parameter, and with the feedback information from the servos, true closed loop positional and velocity control of these servos can easily be achieved. SV

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